The relationship, in my mind, is that I valued their livelihood and the service they'd provided me; what I'm saying is that some people may feel the same about Twitterific. I'm not arguing that anyone should abstain from requesting a refund, I'm illustrating why they might.
I don't object to a customer voluntarily deciding to toss a "parting tip" to Twitterific on their way out. I object to Twitterific not, by default, offering pro-rated refunds.
(It does seem from other responses here that doing so is just not possible on Apple's payments platform. That's pretty lame of Apple, IMO, but oh well.)
But we're talking about a company that provided a service that people were paying for, and now no longer provides that service. Any unused portion of any subscription bought should absolutely be refunded. It's actually a bit gross that the company is not pro-actively refunding subscriptions. They made a bet that Twitter's API would always remain available to them; it's not on their customers to bear any cost for the failure of that bet.